A certain air pressure control mechanism relating to a compressed air brake system is shown and disclosed in instruction and service data literature SD-36 REV. Sept. 18, 1968 by Bendix-Westinghouse. In this particular system a switch or change-over valve, which is also known as a governor, was located in the pressure line. The pneumatic control outlet of the governor is connected with the pneumatic control connection of the compressor for the purpose of opening the suction valve to establish the unloading phase of the compressor.
During the unloading phase, the air pressure of the consumer is supplied to the control connection of the compressor which acts upon a piston to open the suction valve.
With this cutting-off technique, the pressure generating power of the compressor is largely eliminated because the suction valve is kept open during the unloading or idling phase of the compressor. However, relatively high temperatures can result due to the fact that no exchange of air takes place, and since the piston is still moving the hot air is pumped back and forth between the suction and compression chambers to further increase heating even more. Since there is no existing exchange of the air which is moving back and forth, this leads to a relatively long lag time in which unwanted oil is pumped from the crankcase into the suction and compression chambers which subsequently leads to oil carbon formation.